Blender 2.8 - What is 'Tweak' in Preferences > Keymap
Tweak is for when a mouse button needs to be pressed and held, followed by dragging the mouse. You can find this information in the docs of Blender 2.8.

A good example for this is SHIFT + Right Click. As long as you keep the right mouse button pressed, you can drag the 3D cursor around.

I couldn't find a particular purpose of having transform.vertex_random set to tweak, it doesn't seem to react to mouse drags. Perhaps that's a misconfiguration.